Understanding relationship between air pressure and flow

“Pressure” and “flow” are common terms when discussing air compressors, but the relationship between the two is often misunderstood.

Pressure, typically measured in psi, determines an air compressor’s ability to perform a certain amount of work at any given point in time. Consider an application as simple as sliding a wooden block across a table with compressed air. In this application, 75 psi (5.2 bar) of air might not deliver enough force to move the block, but 100 psi (6.9 bar) would.

Figure 1 (left). Pressure is typically measured in psi and determines an air compressor’s ability to perform a certain amount of work at a given point in time. Figure 2 (right). Flow is typically measured in cubic feet per minute at a specific pressure and indicates an air compressor’s ability to continuously perform a task. Image courtesy of Atlas Copco.

At a minimum, the compressor must provide the pressure (force) needed to perform the job. If the minimum pressure required is 100 psi but the compressor can’t provide it, the job won’t get done. However, too much pressure can damage compressed air equipment or cause it to operate erratically. Each pneumatically powered device has a stated pressure range in which it works reliably, and these limits must not be exceeded.

Flow is the ability of an air compressor to continuously perform a task. In the U.S., flow is typically measured in cubic feet per minute (cfm) at a specific pressure. The amount of flow depends on the length of time needed to complete the task.

For example, if you only had to move the previously mentioned block a short distance every hour, a small compressor with a compressed-air storage tank would be suitable. The compressor would refill the tank, storing the air until the next time it was needed to move the block. However, if the process required the block to continuously move during a 24-hour period, a larger compressor with continuous flow would be needed.

Insufficient flow will require breaks to be taken during the process while the compressor builds up pressure in a reserve air tank. Frequent breaks to let pressure build or pressure that builds slowly indicates that air may be leaking from the compressed air system.

Pressure is determined by the job being performed. For instance, spraying paint requires significantly less pressure than bottle blowing. Flow is determined by how many of these jobs must be performed simultaneously or how often they must be performed. 

For a given compressor, such as a 50-hp (37.3kW) one, flow will decrease as pressure increases, and vice versa. Therefore, it’s important to purchase a compressor that optimizes flow and pressure.

And keep in mind that every 2 psi (0.14 bar) increase in pressure results in 1 percent more energy needed to maintain the same airflow (cfm).